1. Field of the Invention
The present invention relates generally to ultrasonic motors, and more specifically to an ultrasonic motor of the type wherein longitudinal and torsional vibrations are combined to produce a standing wave pattern that gives a rotational drive of high torque.
2. Description of the Related Art
As illustrated in FIG. 1A, a conventional ultrasonic motor of the longitudinal-torsional composite resonator type comprises a stator 10 for providing torsional and longitudinal vibrations and a rotor 11 driven by the stator in a direction determined by the timing of an instantaneous longitudinal distortion occurring with respect to torsional distortions of a predetermined direction. If the stator is given an upward thrust at the same time it is given a clockwise distortion, the torsional thrust is transmitted to the rotor, giving it a clockwise rotation. A ring-shaped, planar support member 12 is secured between the torsional and longitudinal vibrators of the stator 10. Support member 12 is formed with screw holes 13 (FIG. 1B) to mount the motor on a stationary body 14 using screws 15. Because of the high stiffness of the support member 12 on a plane perpendicular to the longitudinal axis of the stator, the torsional vibration of the stator is undesirably constrained in terms of operating efficiency. If the thickness of the support member 12 is thinner than a certain value, it no longer acts as a constraint on the longitudinal vibration of the stator. However, if one of the antiresonant frequencies of the bending vibration of the support member 12 in the direction of its thickness matches the frequency of the stator's longitudinal vibration, it would not only act as a constraint on the torsional vibration but on the longitudinal vibration. The efficiency of an ultrasonic motor with a diameter of 15 mm using the conventional support structure is below 15%.